We have allowed gravity to take us on a tour of the Universe in the first half of this book. It has taken us from the planet Earth to the rest of the Solar System, then to other stars, and from there to galaxies. Gravity wants to lead us further, because we have not yet come to understand its most profound consequences. These include black holes, which we met briefly in Chapter 4, and the Big Bang, which is the beginning of time itself.

These matters require strong gravity: gravity that is strong enough to trap light in a black hole or to arrest the expansion of the entire Universe. Studying strong gravity takes us beyond the limits where we can trust Newton's theory of gravity and his laws of motion.